Deep Space Nine wasn't just a darker version of Star Trek in a metaphorical sense — it was also lit very differently. The Trek Companion podcast has an exclusive interview with Kris Krosskove, who was camera operator and often director of photography on both TNG and DS9. And there's a full transcript at TrekWeb.

Here's how he describes the lighting on The Next Generation:

Well, we looked at through muslin over the ceiling and it had some plexiglass up there as well, so you were able to get a nice ambient light coming down from the top, but then we went with very large sources which helped round things out and then anytime we shot Michael Dorn and what we called the turtle head, you get a little stronger top light which would then accentuate those bumps that he had on that large forehead, so that gave a little more…

And here's how the sets were lit on Deep Space Nine:

Advertisement

Because you could have light depravation after a while, especially on Deep Space Nine with dark, dark, gray sets and low lights and contrast... It was actually a choice that was consciously made to make it darker and grittier. They were not as pristine a ship as the enterprise was. And, I think that the characters and the places that they traveled…it was a darker, a little seedier environment than the pristine world the enterprise was (in).

He notes that even when they went to a planet on DS9, it was usually not a place with a lot of sunlight.